The LTE-Advanced relay technology requires relay stations to provide backward compatibility for user equipments (abbreviated as UE) of Release 8, that is, LTE-Advanced relay scheme should support the legacy UE of Release 8 without any change to UE, in such case, there are some technical challenges accordingly:
Firstly the legacy UE is designed without consideration of relay, thus it can not be aware of the existence of relay station, which adds difficulty to the design of relay of LTE-Advanced.
Secondly, as relay concept is introduced, some uplink/downlink subframes or timeslots have to be reserved for Backhaul resources between base stations and relay stations. When relay stations receive data from base stations, such subframes or times lots are kept blank, which can not be used to transmit any symbols or data to UE, while present Release 8 assumes each subframe contains reference symbol, and UE estimates channels in a whole time-frequency grid by interpolating/averaging a plurality of reference symbols and reduce noise in channel estimation. It is expected that when UE measure reference symbols at downlink subframes which is preset to be blank, the zero value will be measured, which will result in error or failure of the whole channel estimation.
Thirdly, under the HARQ (Hybrid Automatic Repeat reQuest) mechanism, data transmission and ACK/NACK feedback have fixed timing relation. Because some uplink/downlink sub frames or timeslots are reserved for backhaul and cannot be used by UK, it may cause that after sender transmits codeword, no transmission of corresponding ACK/NACK feedback at anticipated subframes or timeslots is sent by the receiver side and no feedback is received at sender side consequently. The ACK/NACK collision may cause inefficient operation such as unnecessary retransmission. Such frequent “non-access” subframes lead to great difficulties to retain the fixed timing relation between HARQ transmission and ACK/NACK feedback. Furthermore, in each radio frame (10 ms for a period), the subframes #0, #4, #5 and #9 in FDD system and #0, #1, #5 and #9 in TDD system carry P-BCH (primary broadcast channel), P-SCH (Primary Synchronization Channel), S-SCH (Secondary Synchronization Channel), etc. for UE and thus can not be used for relay backhaul link, such limitation causes more difficulties to maintain HARQ timing.
At present there is a fake MBSFN subframe relay method, the downlink part of which is illustrated in FIG. 1. Wherein, during backhaul, RN will configure some downlink subframe as downlink MBSFN subframe toward UE and receive the data information from eNB in the subframe. UE does not use the reference signal received from the MBSFN subframe for channel estimation between eNB and UE, thus RN can communicate with eNB by these subframe, which does not affect channel estimation of UE. When communicating with UE, RN transmits information of PDCCH (Packet Dedicated Control Channel) and PHICH (Physical HARQ Indicator Channel) information on the basis of cell-specific reference signals of the RN (abbreviated as RN CRS), which causes legacy UE identifies the RN as eNB and receives data signal which is accompanied by RN CRS. RN blanks some uplink subframe by proper scheduling for uplink transmission, while both backhaul link and access link run with a same existing TDD uplink/downlink configuration, i.e., the same downlink subframe and uplink subframe allocation, the same HARQ timing and the same downlink grant association.
The fake MBSFN subframe relay method has some defects (R1-090827): firstly, UE does not estimate the subframe configured as MBSFN subframe, which affects the continuity and integrality of the estimation. Secondly, TDD time slot configurations will affect the efficiency of access link and performance of communication of the method, for instance, when an uplink subframe is used for the backhaul link transmission from RN to eNB, the subframe in access link must be configured as “MBSFN” and be blanked. Consequently, after downlink data is transmitted in downlink subframe, the feedback of downlink data HARQ from UE in uplink subframe cannot be received by RN, which is expected to be received by RN originally, such HARQ feedback loss will lead to the waste of time-frequency resource, i.e. the downlink subframe can not transmit PDSCH. TDD configuration 4 is considered as an example, as illustrated in FIG. 2, wherein the HARQ feedback of subframes #0, #1, #5 and #6 are carried either on subframe #2 or #3 according to Rel-8 specification. When the fake MBSFN subframe method is in use, since at least one of subframe #2 or #3 has to be used by the backhaul link, HARQ feedback from at least one of UEs cannot be received by the RNs which uses subframe of #0, #1, #5 and #6 to transmit downlink data.
In addition, TDD configuration 0 and configuration 5 are not applicable to TDD relay, because sub-frame #0, #1, #5 and #6 cannot be used for MBSFN due to P-BCH, P-SCH and S-SCH for configuration-0, and the only uplink sub-frame #2 cannot be occupied for configuration-5.
Furthermore, R1-091403 provides a method of data retransmission assisted by RN: base station sends data signal to UE directly, simultaneously RN monitors the data signal sent from base station and demodulates and decodes the data signal, then restores original information; when UE demodulates and decodes the data signal received from base station incorrectly, base station will retransmit the data signal to UE according to HARQ rule, simultaneously RN generates the data signal which is corresponding to the original information and has the same format as sent by base station, then the data signal will be sent to UE, the data signals sent by base station and RN overlap in space, which causes UE demodulates and decodes original information properly with much larger probability.
The solution has some defects (3GPP RAN1 #56bis, March 2009): the solution is limited to assist HARQ to retransmit data only, rather than obtaining relay gain by high order modulation and coding of relay link of a relay communication method in general, moreover, only less than or equal to 30% of HARQ service requires retransmission according to statistics, so the utility efficiency of the solution is lower. In addition, as for relay wireless network, channel quality between base station and UE is bad usually; while in R1-91403 solution, base station must communicate with UE by bad channel, in the scenario low order coding and modulation has to be assumed, therefore the communication between RN and UE must assume low order communication mode which is employed by base station also, thus the capacity based on the communication method is lower.